Field
The invention relates to endoscopes and in particular to a relay set for an endoscope with a plurality of relay sets of the same type, comprising two plano-convex rod lenses which face one another with their planar end surfaces, and an achromat that is arranged between these rod lenses, particularly in a central aperture plane of said relay set, wherein said achromat is designed as an arrangement of at least two lenses which have different refractive indices and Abbe numbers, and is located at a distance from the rod lenses.
Prior Art
Corresponding endoscopes affected by the invention include rigid endoscopes, which have an opening with objective lenses on their distal tip, through which light from an operative field can enter the endoscope. Endoscopes normally also have light conductors or optical fibers next to the rod lens system, with which light from a light source on the proximal end of the endoscope is directed to the distal end in order to illuminate the operative field. Since the relay sets and the optical fibers share the existing space in the endoscope, a compromise must be found between the diameter of the lenses of the relay sets and the available cross-sectional surface for light conductors in order to achieve a maximum image brightness.
Rod lens systems with several relay sets of rod lenses transfer the perceived image to the proximal end of the endoscope, where it is received by an operator through an eyepiece or by means of an image sensor. Since the respective image is inverted in the image planes between the relay sets or respectively rotated by 180°, an odd number of relay sets is generally used. The rod lenses are used in order to direct as much of the light as possible to the eyepiece of the endoscope.
A rigid endoscope with for example 3, 5 or 7 relay sets, which in turn consists respectively of several rod lenses and additional lenses lying in between, thus has a high number of boundary surfaces with air or vacuum gaps or lenses connecting to it with different optical properties. A correction of image errors, called aberrations, is thereby possible. This includes spherical aberration, coma, astigmatism, image field curvature, distortion and color errors, the so-called chromatic aberrations.
Chromatic aberrations result due to the dispersion of optical glasses in that light of different wavelengths is deflected to different degrees. The refractive index of the glass is not a constant but rather a wavelength-dependent function. Simply put, the dispersion describes the steepness of the progression of the refractive index of an optical material.
Axial and lateral chromatic aberrations result from the different degrees of light refraction of different wavelengths of the light. The axial chromatic aberration describes the effect that, in a focusing system, the distance between the respective focus point and the lens system depends on the wavelength. The lateral chromatic aberration describes the wavelength dependency of the enlargement of an object in the image plane. The chromatic aberrations can be partially compensated or reduced through use of lenses with different materials. Thus, the axial chromatic aberration for two wavelengths can be corrected with an achromat. In the case of apochromatic optical systems, the axial chromatic aberration is corrected for three wavelengths. However, the aberration for the wavelengths which are not corrected exactly is thereby reduced as well.
In most cases, the relay sets used in the endoscopes of the applicant of the present patent application have two plano-convex rod lenses which face one another with their planar end surfaces. An achromat, i.e. an optical subassembly made up of several lenses, with which chromatic aberrations are at least partially compensated, is arranged in the gap between the rod lenses. The achromats are usually made up of two or three lenses with different optical properties, above all a different refractive index and different Abbe numbers. The Abbe number V, which is determined as
                    V        =                                            n              e                        -            1                                              n                              F                ′                                      -                          n                              C                ′                                                                        (        1        )            within the framework of the present application from the wavelength-dependent refractive index n of the material, is a measure for the dispersion of the material, wherein a low Abbe number stands for a high dispersion and a high Abbe number stands for a low dispersion. The indices e, F′ and C′ named in formula (1) stand for the Fraunhofer lines e (light source mercury, wavelength 546.074 nm), F′ (cadmium, 479.9914 nm) and C′ (cadmium, 643.8469 nm).
In high-quality optical systems, such as e.g. objectives for reflex (SLR) cameras, lenses made of glass with a particularly low dispersion are used to correct chromatic aberrations. In different contexts, these glasses, depending on their Abbe number and on the context, are also called “special low dispersion glass” (SLD glass), “extraordinary low dispersion glass” (ELD glass), “extra-low dispersion glass” (ED glass) or “ultra-low dispersion glass” (UL glass).
In the context of the present invention, these glasses are collectively referred to as “ED glasses”. ED glasses to be used within the framework of the invention have an Abbe number of 75 or greater. Fluoride glasses for example have Abbe numbers of approximately 77 or 80 or more. The limits are not clearly defined; different manufacturers offer different ED glasses with different Abbe numbers, which lie for example between 77 and 95. At the same time, such ED glasses have a relatively low refractive index of approx. 1.4 to 1.6 compared to optical glasses.
Since ED glasses are fluoride glasses and other special glasses, which are sensitive to humidity and are also considerably more brittle in their mechanical properties than optical glasses, they are very difficult to handle. Moreover, they are comparatively and considerably more expensive in their production and procurement than optical glasses with lower Abbe numbers. Thus, optical lens systems, which are corrected chromatically, normally just have one lens made of ED glass.
It is also necessary in the case of endoscopes with rod lens relay sets to correct chromatic aberrations. This is done with achromats that have combinations of optical glasses made of crown glass with a comparatively low dispersion and flint glass with a comparatively high dispersion. However, the Abbe numbers of these glasses are lower than those of ED glasses. In known endoscopes of the applicant, a mirror symmetrical triplet of lenses around a central plane is used as the achromat, wherein a central biconcave lens made of crown glass is framed by two biconvex lenses made of flint glass.
Since each of the three to seven relay sets already has at least four or five lenses and lenses for the objective and an ocular are also added, such optical systems for rigid endoscopes are complicated to calculate and to optimize since many different parameters must be set and optimized simultaneously. The replacement for example of the material of a single lens of the optical system generally leads to a strong change in the overall optical properties of the system so that a complete reoptimization is necessary.
U.S. Pat. No. 7,733,584 B2 describes an endoscope that is equipped with an objective and three relay sets, one of which is equipped with plano-convex rod lenses, on the concave boundary surface of which is cemented respectively a bi-convex ED lens. The other relay sets do not contain ED glasses. The chromatic aberration of the entire system is corrected with this pair of rod lenses/ED glass lenses combinations. The two additional relay sets do not contain ED glass lenses and are each designed differently from each other and thus respectively individually.
Due to the fact that only three relay sets are used according to U.S. Pat. No. 7,733,584 B2, the already considerably large number of boundary surfaces and materials to be counted is kept relatively small. This concept reaches its limits for longer endoscopes with a greater number of individual relay sets.